How to place any object on the 'floor' of a scene?
You could select a vertex, edge or face in Edit Mode and snap the 3D cursor to the selection Shift-S Cursor to Selection 



In object mode set the object origin to 3D-cursor.



After that you can easily position the object to the 3D cursors position which can be adjusted in the proerties panel.



Followed by Snap Shift-S  Selection to Cursor